Isotope tracer approaches for characterizing artificial recharge and demonstrating regulatory compliance

Potable reuse of groundwater from wastewater origins requires new methods to quantify proposed regulatory criteria such as subsurface residence times, dilution, and water quality transitions. Isotope tracers oxygen-18 ({sup 18}O), tritium ({sup 3}H), dissolved noble gases, and radiocarbon ({sup 14}C) have been used together in Orange County to age-date groundwater, quantify mixing, and characterize changes in total organic carbon (TOC). Simultaneous measurements of {sup 3}H and helium-3 ({sup 3}He) are used to determine groundwater ages between 1 and 40 years with uncertainties of plus/minus one year. These ages map preferred groundwater flowpaths and identify groundwater ages of less than or equal to 1 year. Wells recharged from the Anaheim Lake spreading basin were used to monitor arrival times and dilution of 6000 acre-ft of {sup 18}O-distinct Colorado River (COR) water introduced during a controlled recharge experiment. In addition, isotopically enriched Xe was introduced into the basin to quantify COR dilution of greater than 90%. The COR arrived at 7 wells between 30 and 200 days after recharge commenced. The COR was diluted up to 90% at distances and depths less than 1000 feet from the lake. Results suggest that dilution of 50% is obtained within 6 months from time or recharge. {sup 14}C measured in TOC of Anaheim Lake bottom water was 3 pmc higher than the DOC. The same water collected one month later in a nearby monitoring well, as confirmed by {sup 18}O, showed a 50% reduction in TOC concentration, and a 7 pmc decrease in {sup 14}C relative to the surface water. This result suggests that older carbon components increase in TOC after recharge